1. Field of the Invention
The present invention relates in general to an apparatus and a method for forming an image, and more particularly to an image forming apparatus of an electrophotographic process and a controller therefor.
2. Related Background Art
FIG. 12 shows a color image forming apparatus 10 including image forming means for four colors, yellow Y, magenta M, cyan C and black K. Note that symbols a, b, c and d which are added to respective reference numerals in the figure correspond to yellow Y, magenta M, cyan C and black K, respectively. The color image forming apparatus 10 includes photosensitive drums 1a, 1b, 1c and 1d for forming respective electrostatic latent images, and motors 6a, 6b, 6c and 6d for driving the respective photosensitive drums 1a, 1b, 1c and 1d. 
Laser scanners 2a, 2b, 2c and 2d for carrying out exposure in correspondence to an image signal to form electrostatic latent images on the photosensitive drums 1a, 1b, 1c and 1d are disposed above the photosensitive drums 1a, 1b, 1c and 1d, respectively. In addition, the image forming apparatus 10 includes a conveyance belt 3 for successively conveying a sheet to image forming parts for the respective colors, drive rollers 4 connected to drive means having a motor, a gear and the like in order to drive the conveyance belt 3, a motor 6e for driving the drive rollers 4, and a fixing device 5 for melting and fixing toner transferred onto a sheet.
Data of an image to be printed is transmitted from a personal computer (PC) to a printer. When the image formation corresponding to a system of a printer engine is completed, and then an operation state becomes a state where the printing can be carried out, a sheet is fed from a sheet cassette. When the sheet arrives at the conveyance belt 3, the sheet is successively conveyed to the image forming parts for the respective colors by the conveyance belt 3. Image signals of the respective colors are sent to the respective laser scanners 2a, 2b, 2c and 2d in correspondence to a timing at which the sheet is conveyed by the conveyance belt 3 to form electrostatic latent images onto the respective photosensitive drums 1a, 1b, 1c and 1d. The electrostatic latent images are then developed with toners by developing devices (not shown) to be transferred onto the sheet in transferring parts (not shown), respectively. In the figure, the images are formed on the sheet in order of yellow Y, magenta M, cyan C and black K. Thereafter, the sheet is separated from the conveyance belt 3, and the toner image is then fixed onto the sheet by the heat in a fixing device to be discharged to the outside.
Incidentally, in the multi-color image forming apparatus constructed as described above, differences in the image formation positions of the respective colors appears as misregistration in the image to cause degradation of the image quality. The misregistration can be roughly classified into a stationary misregistration generated due to the position shift when assembling the developing devices of the respective colors (hereinafter referred to as “D.C. misregistration”), and a periodic misregistration generated due to the deviation of shafts of rotation members (hereinafter referred to as “A.C. misregistration”).
As for measures taken to cope with the A.C. color drift, there is known a method described in Japanese Patent Application Laid-Open No. 2001-022147. Thus, a technique for individually controlling rotation phases of the rotation members of the respective colors is known. However, the above-mentioned method has the following disadvantages. That is, a first print time becomes long all the more since a phase adjustment sequence is necessarily carried out whenever the motors are activated.